Field of the Invention
The present invention relates to a control device for a motor drive device that performs motion control of a moving member via a transmission mechanism connected to a motor, a control device of a multi-axial motor drive device where multiple motor drive devices are serially connected, and a control method for the motor drive device.
Description of the Related Art
As of recent, robots are being developed to perform various tasks instead of humans. To realize robots that can perform precise and speedy work like human hands, both high precision and high speed must be realized in robot actions. Such robots use motors as the power source to drive joints. Many use transmission mechanisms such as reducers, ball screws, and so forth, to increase motor thrust and convert rotary movement into linear movement.
However, transmission mechanisms have inherent factors that impede increased precision. Examples include twisting, angle error due to meshing of cogs in reducers, backlash, friction, minute vibrations of small parts, angle error due to deformation of ball screws, resonance of screws, lost motion, and so forth. Accordingly, even if the motor is driven with precision, the precision of the robot actions deteriorates due to the above-listed factors.
Also, transmission mechanisms are more flexible in comparison with structures such as robot frames, and accordingly twist and behave like elastics. Accordingly, a moving member regarding which movement is to be controlled, that is connected to the transmission mechanism, exhibits single harmonic motion as to the motor, which also causes deterioration in precision. Further, reactive force of the single harmonic motion acts on the motor, resulting in deviation of motor position, which causes even further deterioration in precision.
There are two general methods to control motors for robot joints and the like, semi-closed control (e.g., see Japanese Patent Laid-Open No. 61-201304) and full-closed control (e.g., see Japanese Patent Laid-Open Nos. 7-225615 and 2011-176913). Semi-closed control is a method where the position of the output shaft of the motor (output member) is detected, and the detection results are fed back to a position command of the motor. Full-closed control is a method where the position of a moving member that is subjected to moving drive by the motor via a transmission mechanism such as a reducer or the like, and the detection results are fed back to a position command of the motor. Generally, semi-closed control has quick action speed but low positional precision, while on the other hand full-closed control has highly positional precision but slow action speed. Thus, it can be said precision and rapidity are in a tradeoff relationship.
It would seem possible to prevent positional deviation of the motor in semi-closed control by output thrust that counters the counteractive force. However, if the motor continues to output thrust countering the counteractive force and there is no positional deviation, feedback cannot be performed, so as a result, the single harmonic motion continues unchecked. Once single harmonic motion becomes sustained, the single harmonic motion generated each time twisting occurs in the transmission mechanism that is operating is amplified, and may become mechanical resonance.
Accordingly, positional deviation of the motor due to reactive force from the elastic force of the transmission mechanism cannot be prevented even in semi-closed control, if the operating speed (gain) is raised. If positional deviation of the power source due to reactive force from elastic force at the transmission mechanism is to be prevented, the operating speed (gain) has to be lowered and driving control of the motor performed so that no single harmonic motion occurs, even in semi-closed control.
Accordingly, Japanese Patent Laid-Open No. 61-201304 proposes semi-closed control where a motion equation is prepared beforehand that takes into consideration the mechanical rigidity of the transmission mechanism, and the calculation results thereof are added to the command value of the semi-closed control. This aims to realize both precision and rapidity.
On the other hand, Japanese Patent Laid-Open No. 7-225615 proposes full-closed control where the position of the moving member is detected by a sensor and subtracted from a target value, the position of the motor is corrected according to the different in position, and the position of the moving member is made to copy the target value. Japanese Patent Laid-Open No. 2011-176913 proposes full-closed control where the difference between the position of a vibrating moving member and the position of the motor is obtained, a torque value is calculated taking torsional stiffness into consideration, and controlling so that this matches a torque target value.
The semi-closed control in Japanese Patent Laid-Open No. 61-201304 is capable of high-speed operations taking advantage of the characteristics of semi-closed control. However, prediction of transmission error that changes according to temperature variation and over time is difficult, and accordingly there is a problem that satisfying the required precision is difficult.
In the other hand, the full-closed control in Japanese Patent Laid-Open No. 7-225615 has a problem that vibration phenomena cannot be measured, and accordingly, increasing operation speed (gain of motor correction) results in oscillation. The full-closed control in Japanese Patent Laid-Open No. 7-225615 thus has a problem that operation speed (positional correction) is sluggish.
Japanese Patent Laid-Open No. 2011-176913 has been conceived to raise speed in full-closed control. The full-closed control in Japanese Patent Laid-Open No. 2011-176913 employs a configuration where the primary factor for sluggishness in positional correction has been eliminated, so it had been thought that both precision and rapidity could be realized in robot actions. However, full-closed control differs from semi-closed control with regard to the point that a transmission mechanism exists between sensor and motor, and transmission mechanisms are characterized by having backlash, friction, and natural vibration at frequencies higher than the above-described single harmonic motion. The full-closed control in Japanese Patent Laid-Open No. 2011-176913 also has backlash, friction, and natural vibration at frequencies higher than the above-described single harmonic motion, and accordingly there has been the problem that increasing operation speed (gain of motor correction) results in oscillation. Accordingly, there has been an issue that gain cannot be sufficiently raised even in the full-closed control in Japanese Patent Laid-Open No. 2011-176913 since high-order vibrations will occur, and consequently high-speed driving cannot be performed.